In many situations, devices must operate in potentially hazardous conditions, such as where a fuel mixture may be ignited by uncontrolled operating or environmental conditions. For example, vehicles, including aerospace vehicles, typically operate with a fuel that must be maintained in a safe condition during storage and use. The ignition hazard should be minimized even when the vehicle is subject to uncontrolled events such as an accident, electrical malfunction, a lightning strike, or static electrical discharge. Other applications requiring ignition hazard consideration include fuel transport, fuel storage, mining operations, chemical processing, metal fabrication, power plant construction and operation, and operations which involve combustible particulate such as sawdust, metal, flour, and grain.
In the aerospace industry, lightning strikes of aircraft are a concern because they could result in electrical arcs and/or heating sufficient to ignite vaporous fuel mixtures. Though lightning passes through aircraft virtually always without resulting harm, newer aircraft designs incorporating composite materials include less metal and conductors to shunt and/or dissipate the energy of a lightning strike.
During a lightning strike on an aircraft, a high electrical current may propagate through conductive paths on the aircraft. Due to the non-isotropic electrical conduction of composite materials used in certain aircraft designs and potentially poor electrical connection at panel interfaces, the electrical current may pass through a fastener stack when propagating from one composite panel to another. While passing through a fastener stack, the current may generate electromagnetic effects, such as electrical arcs, hot gas, and/or hot particles, that may interact with combustible fuel vapor (in the absence of safety measures). Such an uncontained emission of energy may pose an ignition risk to aircraft fuel tanks. In a typical commercial aircraft, hundreds to thousands of fastener stacks may extend into the fuel tank, and every one needs to not pose an ignition risk in the event of a lightning strike.
As a safety measure, the exposed ends on fastener stacks may be covered with polysulfide cap seals to seal fuel mixtures in a fuel tank from any arcs, hot gas, or hot particles that may form as a result of a lightning strike. However, these caps require an airtight seal to be effective. Without an airtight seal, fuel may contact the fastener stack and/or the arc, hot gas, or hot particles may bypass the unsealed cap to present an ignition risk. Additionally, environmental exposure (e.g., thermal cycling) and/or electromagnetic effects at the fastener stack may damage the seal. Achieving a resilient, airtight seal is a labor-intensive process that may need to be repeated thousands of times per aircraft. The associated installation time as well as inspection time increase the cost and production time of aircraft.